Solid electrolytic capacitors comprise an anode body of a valve metal such as Al (aluminum) or Ta (tantalum), a dielectric oxide film formed on the anode body by an electrolytic oxidation treatment, and a cathode layer formed by depositing MnO.sub.2 (manganese dioxide), conductive organic compound or like solid conductive substance formed on the oxide film in intimate contact therewith. The term "valve metal" as used herein refers to a metal which forms a highly compacted durable dielectric oxide film when subjected to an electrolytic oxidation treatment. Such metals include Ti (titanium) and Nb (niobium) in addition to Al and Ta. Since the dielectric oxide film has a very small thickness, it is possible to prepare electrolytic capacitors having a smaller size and a greater capacity than other capacitors such as paper capacitors and film capacitors.
With reference to FIG. 1, a solid electrolyte capacitor 1 in the form of a chip is prepared by attaching an anode lead wire 21 to an anode body 2 of valve metal, forming a dielectric oxide film 20 on the surface of the anode body 2 by an electrolytic oxidation treatment and depositing a solid conductive substance on the oxide film 20 in intimate contact therewith to form a cathode layer 3, whereby a capacitor element 11 is obtained. Next, a capacitor body 10 is prepared by coating the cathode layer 3 of the capacitor element with a carbon layer 4, and coating the carbon layer 4 with a silver paste layer 5. As shown in FIG. 2, a lead frame 6 is subsequently bonded to the silver paste layer 5 with a silver adhesive 52, and a lead frame 60 is joined to the anode lead wire 21 as by welding. As shown in FIG. 3, the capacitor body 10 and portions of the lead frames 6, 60 are encapsulated in a resin as by injection molding to form a package 7, followed by aging. The solid electrolyte capacitor 1 is thus completed.
As compared with other types of capacitors such as ceramic capacitors, electrolyte capacitors have a higher ESR (equivalent series resistance), which it has been desired to improve. The ESR of the electrolyte capacitor is the sum mainly of the internal resistance of the cathode layer 3, internal resistances of the lead frames 6, 60, contact resistance between the capacitor body 10 and the lead frames 6, 60 and the contact resistance between an external circuit and the lead frames 6, 60. It has heretofore been required to reduce the internal resistance of the cathode layer 3 since this resistance is exceedingly greater than the other resistances.
Recently use of a conductive organic compound such as polypyrrole, polyaniline or TCNQ (7,7,8,8-tetracyanoquinodimethane) complex salt for the cathode layer 3 has remarkably reduced the internal resistance of the cathode layer 3. To reduce the ESR of solid electrolyte capacitors, it is therefore required to lower the internal resistances of the lead frames 6, 60 and the above-mentioned contact resistances in addition to the internal resistance of the cathode layer 3. Of these, the resistances relating to the present invention will be described, i.e., the contact resistance between the capacitor body 10 and the lead frames 6, 60, and the internal resistances of the lead frames 6, 60.
a. Contact Resistance between Capacitor Body and Lead Frames
The present applicant has conducted various tests and found that the contact resistance between the capacitor body and the lead frames 6, 60 is attributable mainly to the contact resistance between the silver paste layer 5 of the capacitor body and the silver adhesive 52, and that this resistance is dependent on the viscosity of the silver paste.
The silver paste contains silver, a binder and a solvent (volatile component) for dissolving the binder therein. The binder is a resin such as epoxy resin. The silver paste layer 5 is formed on the carbon layer 4 usually by dipping the capacitor element 11 coated with the carbon layer 4.
When low in viscosity, the silver paste contains a high proportion of solvent, readily permitting separation between the silver and the binder. When the silver paste of low viscosity is used for forming the silver paste layer 5, therefore, the silver paste becomes surfaced with the binder which is the resin component, increasing the contact resistance between the silver paste layer 5 and the silver adhesive 52.
When a silver paste of high viscosity is used for forming the silver paste layer 5, the contact resistance between the silver paste layer 5 and the silver adhesive 52 becomes small. However, if applied by dipping in this case, the silver paste adheres to the carbon layer 4 to an increased thickness, increasing the size of the capacitor body 10 and affording a solid electrolyte capacitor 1 of increased size. Although the silver paste of high viscosity is applicable to the carbon layer 4 in a small thickness by brush coating, this method is not practically useful in view of uneven application and productivity.
b. Internal Resistance of Lead Frames
A nickel-iron alloy having a nickel content of about 42% (hereinafter referred to as the "42-alloy") is generally used for the lead frames 6, 60 because of good electric conductivity, high strength to support the capacitor body 10 and corrosion resistance.
Since the 42-alloy is lower than gold or copper in conductivity, lead frames made of gold or copper are smaller in internal resistance than those made of the 42-alloy. However, lead frames made of gold or copper which is lower than the 42-alloy in strength need to have a greater thickness than those prepared from the 42-alloy so as to retain strength to support the capacitor. This renders the solid electrolyte capacitor 1 large-sized.
A first object of the present invention is to provide a process and apparatus for producing a solid electrolyte capacitor wherein the contact resistance between the capacitor body 10 and the lead frames 6, 60 is decreased without increasing the capacitor in size.
A second object of the invention is to provide a solid electrolyte capacitor wherein the lead frames 6, 60 are diminished in internal resistance without making the capacitor large-sized.